1. Field of the Invention
The present invention relates to an adaptive equalizing apparatus and method, used in an optical or magnetic recorder to make waveform equalization with a PRML (partial response maximum-likelihood) technique which is a combination of the partial response equalization and the maximum-likelihood decoding such as the Viterbi decoding, FDTS (fixed delay tree search) or the like.
2. Description of the Related Art
In a recorder using a superdense recording type optical disk such as the recently developed blue-ray disk (brand name), the playback system thereof is regarded as a transversal filter to make waveform equalization by the PRML technique which is a combination of the partial-response equalization (will be referred to as “PR equalization” hereunder) utilizing an inter-symbol interference which will take place in the transversal filter, and the maximum-likelihood binarization such as the Viterbi decoding, FDTS (fixed delay tree search), etc. which can prevent any random noise-caused degradation of the signal-to-noise ratio (S/N ratio).
Also, there is well known an adaptive Viterbi decoding capable of decoding, with a high performance, even an input waveform including an nonlinearity such as an asymmetry or the like (as in the Japanese Patent Application Laid Open No. 261273 of 1998, which will be referred to as “patent document No. 1) and Naoki Ide, “Adaptive Partial-Response Maximum-Likelihood Detection in Optical Recording Media” ISOM2002).
Also, to equalize and decode a read signal, it is well known to make nonlinear equalization of an input waveform itself by a nonlinear model and then decode the read signal at a downstream stage.
If a signal supplied to a general linear adaptive equalizer includes a nonlinear component such as a vertical asymmetry in the amplitude thereof, it is theoretically difficult to make any Wiener-optimal adaptive equalization of the input signal and a nonlinear equalization error will remain in a waveform output from the linear adaptive equalizer. It is known that in case it can be presumed that the nonlinear component can be expanded into a Volterra series by an ideal linear signal, approximate adaptive equalization can be made of the nonlinear equalization error by a adaptive equalization Volterra filter whose nonlinear equalization error has an order corresponding to the magnitude of an order of the nonlinearity of the nonlinear equalization error (as in Mathews, V. J., “Adaptive Polynomial Filters” IEEE Signal Processing Magazine, Volume 8, Issue 3, July 1991, pp. 10-26).
It has been pointed out that especially in optical recording, signals to be recorded include a nonlinear component and various signal processing techniques have been researched for improvement of characteristics. In the signal processing method disclosed in the Japanese Patent Application Laid Open No. 2001-525101, nonlinear components are removed by an adaptive equalization Volterra filter from a read signal from an optical disk, then the waveform signal resulted from the removal of nonlinear distortion is supplied to a Viterbi decoder, and an improved error-rate is implemented while reducing the scale of calculation through contrivance of the detector design.